The present invention relates to a device for producing a detection signal when interfering pieces appear in an at least largely homogeneous, in particular a non-conductive stream of material to be conveyed, in which an alternating current generator builds up an alternating electromagnetic field via a transmitting coil in a segment of the stream of material to be monitored, whose changes in amplitude and phase are detected by a coil system, that feeds an evaluating circuit, for deriving the detection signal. Such devices are described in, for example, German Patent Nos. 43 42 826, 195 21 266, and the literature described therein.
These types of devices are needed, for example, to detect metal pieces in a stream of material to be conveyed made up of only paper or recyclable plastic which is being fed to a fragmenting apparatus (shredder) for reprocessing. If the material to be conveyed contains solid metal pieces, this can result in significant disruptions in the work flow, if not in fact in destruction of machine parts, even if the pieces are only small. Depending on its intended use, the conveyor apparatus may be designed for example as a conveyor belt, a vibrating conveyor or the like. The detection signal emitted by the device is used to actuate protective equipment, such as optical and/or acoustical signaling means, and shut-off devices to stop the conveyor system, so that the interfering piece may be removed. Using a resetting device, frequently also referred to as a reset button, the entire system can be put in operation again following a detection signal and the corresponding conveyor stoppage. However, it may occur that after transport is halted, an interfering piece of metal is removed from the material to be conveyed, but another metal piece is still present in this segment of the material to be conveyed and is overlooked. When the reset button is actuated, the conveyor stream starts up again, and the additional metal piece reaches downline machines that need to be protected from interfering portions such as metal pieces.